1. Field of the Invention
The present invention relates to an optical encoder having an improved structure for the optical detecting section thereof.
2. Description of the Related Art
Optical encoders include rotary type encoders which detect the position and speed of a rotational moving object, such as a motor, or the like, and linear type encoders which detect the position and speed of a linear moving object. Of these, one example of an encoder used widely as a rotary type optical encoder is now described with reference to FIG. 23.
A movable code plate 100 is made from a glass plate onto which chrome has been vapour deposited, or a metal plate, such as stainless steel, nickel, copper, or the like, and light transmitting sections 101a, 101b are formed by etching in portions of ring-shaped regions scanned by a light-emitting element (omitted from drawing) of a light source 1. The areas outside the light transmitting sections 101a, 101b form light shielding sections.
A light source 1 and a light receiving section 2 are provided on either side of the movable code plate 100. The light source 1 and light receiving section 2 are constituted respectively by a prescribed number of light-emitting elements (omitted from drawing) and photorecptor elements 21, 22. When a light transmitting section 101a, 101b is positioned in front of the photoreceptor elements 21, 22, light projected from the light source 1 to the movable code plate 100 is transmitted by the light transmitting section 101a, 101b and enters the corresponding photoreceptor element 21, 22, and a signal representing the received light is output by the photoreceptor element 21, 22.
The movable code plate 100 is installed on a rotating shaft 3 (the object of which the rotational position or speed is to be detected), and when it rotates about the axis of the rotating shaft 3, a light reception signal is output periodically by the photoreceptor elements 21, 22. By processing this light reception signal by means of a commonly known type of circuit, which is omitted from the drawing, it is possible to detect the rotational position, rotational speed, or the like, of the rotating shaft 3.
When creating the conventional type of optical encoder described above, firstly, a light-shielding film made from chrome, or the like, is formed onto a movable code plate 100, and the light-shielding film is then removed therefrom by etching in a prescribed code pattern. However, since etching uses chemical substances that require labour-consuming management and processing, a problem arises in that manufacturing costs are high and hence this method is not suitable for mass production.
Therefore, in order to resolve the aforementioned problem, it has been proposed that movable code plates can be manufactured by using an opaque resin material and forming transmitting holes in portions of the region scanned by light from a light source. By adopting movable code plates made from resin of this kind, advantages are obtained in that since no harmful chemical substances are used, manufacturing is simple and inexpensive, and good reproducibility is easy to achieve. However, problems arise in that it is difficult to form fine patterns, since adequate strength cannot be achieved, and therefore it is not possible to obtain the high resolution that can be achieved with code plates made from glass or metal.
Furthermore, as a further method of resolving the aforementioned problems associated with movable code plates made from resin, an optical encoder having a light path changing function has been proposed and a patent application has been made for same (Japanese Patent Application No. 11979/1998 issued as Japanese Patent No. 11-201229, U.S. patent application Ser. No. 09/225,516, European Patent Application No. 199990300085). As illustrated in FIG. 24, this optical encoder comprises a movable code plate 110 wherein the ring-shaped region scanned by a light-emitting element (omitted from drawing) of a light source is divided into a first type of region 110a, 110b (regions indicated by hatching in FIG. 24) through which incident light from the light source 1 is transmitted after being bent at least once, and a second type of region through which incident light from the light source 1 is transmitted directly.
A light-receiving section 2 is positioned in such a manner that the state of light reception by photoreceptor elements 21, 22 in the light-receiving section 2 is different in cases where the light from the light-emitting element in the light source 1 is transmitted through the first type of region 110a, 110b on the movable code plate 110, and cases where the light is transmitted through the second type of region.
In the proposed optical encoder described above, the first type of region 110a, 110b is provided by forming a cut-out section 10 (see FIG. 25) in the movable code plate 110. The cut-out section 10 must be formed to a significant depth in order that the light incident on the movable code plate 110 is bent through a prescribed angle. Therefore, when this type of cut-out section 10 is formed in the regions indicated by the hatching in FIG. 24, thus reducing the material thickness in those regions, the strength of the movable code plate 110 declines accordingly.